When work is performed on an electrical system in a building or facility, it is necessary to trace and identify which circuit interrupter device (e.g., circuit breaker or fuse) is supplying power to a particular power outlet receptacle or electrical component. Manual identification of the fuse or circuit breaker can be accomplished by removing each fuse or opening each circuit breaker, thereby disrupting the power flow through the circuit. Each outlet must subsequently be examined to determine whether the power to the outlet has been disconnected. This method is not only time consuming, but also may not be feasible in situations where it would be hazardous to interrupt the power flow to certain outlets, e.g., in a hospital or in an environment where computers are in use with no backup power.
Alternatively, a variety of circuit testers are available for identifying the fuse or circuit breaker that is supplying power to a particular outlet receptacle. These testers employ an assortment of techniques to distinguish one circuit breaker from the rest. For example, the testers disclosed in U.S. Pat. Nos. 4,906,938 and 5,497,094 use a relaxation oscillator to apply an identification signal comprising a large amplitude current pulse of very short duration to the circuit. A schematic diagram of the transmitter 10 disclosed in U.S. Pat. No. 4,906,938 is shown in FIG. 1. The terminals 12, 14 of transmitter 10 are connected to the outlet or light fixture to be tested. Diode 16 acts as a half-wave rectifier. Specifically, if the voltage across diode 16 is positive, diode 16 acts as a short circuit, and if the voltage across diode 16 is negative, diode 16 acts as an open circuit. Sidac 18 is a short circuit when the voltage thereacross reaches its threshold value of 120-135 volts, and is an open circuit when the current through sidac 18 drops below the minimum holding current of the device. Thus, in this arrangement, sidac 18 acts as a trigger switch.
If a conventional power line voltage is applied to transmitter 10, sidac 18 will initially go into conduction when the line voltage reaches approximately 120 volts. This causes capacitor 20 to immediately charge to the line voltage, resulting in a large amplitude current pulse which is used to identify the circuit. Sidac 18 will continue conducting until the current approaches 0 amps, i.e., approximately 50-150 milliamps, which occurs near the peak of the power line voltage. When sidac 18 is switched off, capacitor 20 will be charged at a voltage level close to the peak voltage, i.e., approximately 150 volts, and can only discharge through resistor 22. Due to the relatively large resistance of resistor 22, the discharge of capacitor 20 will be slow.
Because capacitor 20 remains charged at approximately 150 volts, as the line voltage decreases from 150 volts to 0 volts and continues through its negative cycle, the voltage across diode 16 is negative. Thus, diode 16 remains an open circuit and capacitor 20 continues to discharge slowly through resistor 22.
During the next cycle, diode 16 becomes a short circuit when the line voltage surpasses the charge on the capacitor 20. Sidac 18 will remain an open circuit, however, because the voltage across sidac 18, which is the difference between the line voltage and the voltage across capacitor 20, will not reach its threshold value. Thus, transmitter 10 will not conduct any current until the voltage across capacitor 20 has time to discharge through resistor 22, which does not occur for a number of cycles. This results in a frequency of current spikes less than the power line frequency of 60 hertz.
Other conventional testers introduce a high frequency, low amplitude signal onto the current flowing through the power line. These devices are subject to false indications resulting from spurious signals from transients and switching currents, and require a sensitive receiver to pick up the magnetic field created by the low amplitude current flowing through the circuit. In addition, the capability for discriminating between closely adjacent current-carrying conductors, for example, adjacent circuit breakers in a breaker box, is reduced in these types of testers.
Another drawback associated with the conventional testers described above is that they will not operate if the outlet has been wired incorrectly. Thus, although devices are available which indicate any incorrect wiring, conventional testers cannot be used to identify the fuse or circuit breaker associated with a particular outlet unless the wiring of the outlet is correctly installed.